1. Field of the Invention
The present invention relates to a printer, a facsimile machine, a copying machine and a compound machine of them (hereinafter referred to as an image forming apparatus), which form an output image on a recording medium such as a sheet of paper based on input image data. The present invention also relates to a gradation correction method for correcting the gradations of an output image, and control method of the image forming apparatus.
2. Related Art
In an image forming apparatus based on an electrophotographic system such as a laser printer, printing is performed by undergoing the processes of exposure, development, transfer and fixing. To put it concretely, first, in the exposure process, an electrostatic latent image of an image to be printed is formed on the surface of a photosensitive drum by the irradiation of an exposure beam the light quantity of which is modulated In accordance with the image data of the image to be printed. Hereupon, the light and shade, or the gradation, of the image can be expressed by modulating the intensity of the exposure beam, or by controlling an exposure time. Next, in the development process, toner is blown to the photosensitive drum, and the electrostatic latent image is developed on the photosensitive drum as a toner image. In the transfer step, the toner image is primarily transferred from the photosensitive drum to a transfer belt, and then the transferred toner image is secondary transferred from the transfer belt to a sheet of printing paper. In the fixing process, the printing paper is heated, and thereby the toner image is fixed.
In case of a color printer using a plurality of color toner, the so-called tandem system color printer is well known. In the tandem system color printer, an exposure unit and a development unit are provided for each color. Toner images of respective colors are superposed one upon another on a transfer member such as the transfer belt, and then the superposed toner images are transferred on the printing paper in a lump.
In an image forming apparatus based on the electrophotographic system described above, there are some cases where output density values of the apparatus change owing to the dispersion of respective characteristics of the apparatus and the influences of the environment in which the apparatus is used (the influences of temperature, humidity and the like), and thereby desired output gradations according with input density values cannot be obtained.
Accordingly, a conventional image forming apparatus performs a gradation correction of an output image at every fixed period. As shown in FIG. 13, the gradation correction of the conventional image forming apparatus uses a correcting image including a gradation pattern composed of a plurality of gradations, or patch-shaped density steps changing in density by steps. The conventional image forming apparatus detects output densities of the correcting image which has been transferred on a transfer belt through a photosensitive drum by measuring the reflected light quantities of the correcting image with an optical sensor, and performs the gradation correction of the output image based on measurement results (see, for example, JP-Tokukai-2002-14505A and JP-Tokukai-2001-142266A).
To put it concretely, as shown in FIG. 14, the conventional image forming apparatus calculates a measured curve by plotting measured density values at respective density steps measured by the sensor as output density values to input density values. Then, a correction curve having an inverse characteristic of the measured curve with respect to a straight line indicating aimed output gradations is obtained, and output gradations are corrected based on the correction curve.
However, in the case where the distance from the position where a toner image is developed to the position where the toner image is measured by the sensor on the transfer belt is long, there are some cases where the driving speed of the transfer belt has changed until the density of the correcting image is measured by the sensor owing to deflection of the transfer belt, or mechanical errors of respective members such as a drive motor, gears and the diameters of conveying rollers, each being for driving the transfer belt. As the measurement timing by the sensor, specified timing is previously set in order that density measurements may be performed at correct positions of respective density steps based on the turning speed of the transfer belt, the positions where the gradation pattern is formed, and the patch sizes of the respective density steps. However, when the turning speed of the transfer belt changes, the measurement timing is shifted, and consequently the densities at positions different from the positions where the densities should be measured originally are measured. Because such measurements do not bring about correct output density values to the input density values, erroneous gradation corrections are performed based on inaccurate measurement results.
For solving the problem, it is also considerable that a specific pattern such as a reference line indicating a correct measurement starting position of the sensor is added to the correcting image to correct the shift of the measurement timing of the sensor. However, in that case, a new pattern should be written in the correcting image, and toner for the writing of the pattern is additionally consumed. Consequently, the costs of the apparatus are increased.